Bioassay for estimating the biogenic methane-generating potential of coal samples

https://doi.org/10.1016/j.coal.2008.05.011Get rights and content

Abstract

Generation of secondary biogenic methane in coal beds is likely controlled by a combination of factors such as the bioavailability of coal carbon, the presence of a microbial community to convert coal carbon to methane, and an environment supporting microbial growth and methanogenesis. A set of treatments and controls was developed to bioassay the bioavailability of coal for conversion to methane under defined laboratory conditions. Treatments included adding a well-characterized consortium of bacteria and methanogens (enriched from modern wetland sediments) and providing conditions to support endemic microbial activity. The contribution of desorbed methane in the bioassays was determined in treatments with bromoethane sulfonic acid, an inhibitor of microbial methanogenesis. The bioassay compared 16 subbituminous coal samples collected from beds in Texas (TX), Wyoming (WY), and Alaska (AK), and two bituminous coal samples from Pennsylvania (PA). New biogenic methane was observed in several samples of subbituminous coal with the microbial consortium added, but endemic activity was less commonly observed. The highest methane generation [80 µmol methane/g coal (56 scf/ton or 1.75 cm3/g)] was from a south TX coal sample that was collected from a non-gas-producing well. Subbituminous coals from the Powder River Basin, WY and North Slope Borough, AK contained more sorbed (original) methane than the TX coal sample and generated 0–23 µmol/g (up to 16 scf/ton or 0.5 cm3/g) new biogenic methane in the bioassay. Standard indicators of thermal maturity such as burial depth, nitrogen content, and calorific value did not explain differences in biogenic methane among subbituminous coal samples. No original methane was observed in two bituminous samples from PA, nor was any new methane generated in bioassays of these samples. The bioassay offers a new tool for assessing the potential of coal for biogenic methane generation, and provides a platform for studying the mechanisms involved in this economically important activity.

Introduction

Coal bed methane (CBM), also referred to as coal bed natural gas, is a significant energy resource, accounting for about 10% of natural gas production in the USA (Fletcher, 2005, Petzet, 2005). Natural gas in coal deposits may originate from both thermogenic and microbial processes. Microbial production of natural gas (microbial methanogenesis) from coal generally occurs at shallow depths at temperatures less than 100 °C (Clayton, 1998). Microbial methanogenesis of coal is of particular interest to natural gas producers because it may represent renewable CBM and microbial gas resources are located at shallow depths and are relatively inexpensive to produce. The potential for generating “new” natural gas from coal deposits previously exploited for CBM is of economic interest, especially with the CBM recovery infrastructure already in place.

Primary biogenic gas forms soon after peat deposition and is usually lost prior to burial and coal formation (Scott et al., 1994). Generation of secondary biogenic gas from coal is inferred from carbon isotopic data and coal gas composition (Scott et al., 1994). Little is known, however, about the microbial process of generating methane from coal, or the mechanisms that control the occurrence or rate of the process. Environmental features such as formation water chemistry, coal rank, and regional geology and hydrodynamics have been used as indirect indicators of biogenic methane production potential. For example, Scott et al. (1994) describe the influence of uplift, fracturing, increased groundwater flow, and formation dewatering on the development of secondary biogenic methane. Microbially produced natural gas also appears to be favored in lower-rank coals (lignite, subbituminous coal; Rice and Claypool, 1981), although significant secondary biogenic gas resources are also known to be associated with bituminous coals (Kotarba, 2001, Smith and Pallasser, 1996, Faiz and Hendry, 2006). No studies have yet fully described the microbial community and metabolic pathways involved in generating methane from coal, or the physical and chemical microstructures in coal that predispose its conversion to methane. There are several possible factors that could influence the generation and accumulation of secondary biogenic methane in coal beds, including (1) the bioavailability of carbon in the coal, (2) the presence of a microbial community that is able to utilize the coal carbon, and (3) environmental conditions that support microbial growth and methanogenesis, such as availability of nutrients and a lack of toxic or inhibitory factors (including oxygen).

The investigation reported here specifically addresses the measurement of coal bioavailability under a set of standardized conditions. A microbial consortium was applied to bioassay methane-generating potential in 18 coal samples collected from currently producing, non-producing and previously exploited CBM reservoirs. The standard conditions included bicarbonate-buffered nutrient medium (eliminating the effect of differences in groundwater chemistry), inoculation with WBC-2 (eliminating deficiencies in the coal microbial community), and coal broken but not ground (minimizing destruction of the coal structure). All samples tested were from environments characterized by moderate temperature and low salinity.

Section snippets

Sources of coal

Ten coal samples used in this study were collected from five cored coal gas exploration test wells drilled in the Indio Formation (Wilcox Group, Paleocene to Eocene) of south Texas (1 well) and in the Tongue River Member of the Fort Union Formation (Paleocene) of the Powder River Basin (PRB), Wyoming (4 wells) (Fig. 1; Table 1). Although these samples were not analyzed for standard proximate and ultimate analyses and rank parameters, other coal samples from the Texas well had moisture and ash

Release of original methane from coal

An independent assessment of the release of original methane (methane contained in the coal prior to drilling and sampling) is critical to account for desorption of methane during the bioassay. The amount of original methane released from each coal sample varied and is shown in Table 1. There was no immediate release of methane to the headspace from the Wilcox Group coal (“TX”), and the methane concentration in BES treatments (0.0015 +/− 0.0011 μM for treatments 4 and 5 averaged over the entire

WBC-2 bioassay as a valid measurement of coal bioavailability

In theory, bioassays are experimental methods for assaying the effect of substances on biological communities, but they can also be used to assess biological effects on a substance. In this study, we used a bioassay to assess the bioavailability of organic matter in coal. As designed, this assay reflects both the nature of the coal carbon and the metabolic capabilities of the microorganisms. By applying a standard microbial consortium such as WBC-2, we can eliminate the effects of differences

Conclusion

Under the defined test conditions of the bioassays a small fraction of coal organic matter in many coal samples supported methane generation in the laboratory. Standard methods used to determine thermal maturity such as rank, depth, nitrogen content, and calorific value were not good predictors of biogenic methane production in the assays. The mechanisms involved in secondary biogenic methane generation and the controls on those mechanisms are not yet known. Specifically, it is not known what

Acknowledgements

The authors acknowledge the contributions of the following government agencies and private companies for allowing access to coal samples and other information used in this report: Genesis Gas & Oil LLC, for the Texas samples; Pinnacle Gas Resources Inc., and Huber J M Corporation, for the Wyoming samples; U.S. Bureau of Land Management, Olgoonik Corporation, U.S. Department of Energy, Alaska's Division of Geological and Geophysical Surveys for the Alaska samples; and Pennsylvania Department of

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